1. Field of the Invention
The present invention relates to a main shaft drive for a machine tool. More particularly, the present invention relates to a main shaft drive for a machine tool including a rotation-resistance applying device that applies rotation resistance to a main shaft by causing a pressing surface of a pressing member to press-contact a friction surface of a press member when the main shaft that is rotatably supported at a frame is rotationally driven.
2. Description of the Related Art
Hitherto, as a main shaft drive used in a machine tool, a rotation index table device is known. In the rotation index table device, a circular table on which a workpiece (object to be processed) is placed is secured to a main shaft, and the circular table is rotated by rotationally driving the main shaft. The rotation index table device is used for indexing an angular position of the circular table by rotationally driving the main shaft, and processing the workpiece at the indexed angular position. The rotation index table device of this type includes a clamp device for holding the circular table at the indexed angular position (that is, the indexed position). The clamp device is used for causing the main shaft (circular table) to be in a rotation prevention state at the indexed position.
Ordinarily, with the clamp device being set in a clamped state, the rotation of the circular table is stopped, and a workpiece is processed. However, the workpiece may be finished while continuously rotating the workpiece with the clamp device being set in an unclamped state. In this case, the rotation index table device receives a load from a cutter that processes the workpiece. The load constantly varies even under a fixed processing condition. Therefore, a state of rotation of the main shaft caused by the rotation index table device may be affected by the variation of the load received from the cutter, as a result of which pulsation may occur. The details are as follows.
For example, when a direct drive motor (hereunder referred to as “DD motor”) that directly rotationally drives the main shaft without using a drive transmission means such as a worm gear is used, rotation of the DD motor is controlled while correcting deviation of a rotation angle (rotation amount) of the main shaft, on the basis of the rotation angle (rotation amount) of the main shaft that has been detected. However, since the control of the DD motor may not satisfactorily follow variations in the deviation of the rotation angle caused by the variation of the load received from the cutter, the rotation of the DD motor is not necessarily uniform, as a result of which pulsation may occur in the rotation of the main shaft. When the drive transmission means such as a worm gear is used as driving means of the main shaft, backlash may exist in the worm gear, as a result of which the rotational speed of a driving side of gears that engage each other change due to the variation of the load received from the cutter. In this case, driven-side (main-shaft side) gears rotate independently of the driving side within a backlash range due to inertia, thereby causing pulsation to occur in the rotation of the main shaft.
When pulsation occurs in the rotation of the main shaft, pulsation also occurs in a rotation of the circular table secured to the main shaft. As a result, surface roughness of a finishing surface of a workpiece that is finished while rotating the circular table is increased. As a related art that provides a solution to the problem that such pulsation occurs, a main shaft drive discussed in Japanese Unexamined Patent Application Publication No. 2000-218404 (Patent Document 1) is provided.
Patent Document 1 relates to a lathe serving as a main shaft drive. Since an error in a main shaft angle caused by elastic deformation of a rotation transmission system or backlash of a gear occurs due to variation in a cutting load, a brake device is provided at the main shaft to prevent the error and the backlash from occurring, so that, when a workpiece is processed while rotating the workpiece, the brake device constantly applies a certain load to the main shaft. When an error of the main shaft angle occurs as the cutting load varies, pulsation consequentially occurs in the rotation of the main shaft.
In the case where a certain load (rotation resistance) is constantly applied to the main shaft by the brake device when processing a workpiece while rotating the workpiece, a friction surface of a rotating member and a pressing surface of a brake shoe that slide when they press-contact each other are worn. When the pressing surface of the brake shoe and the friction surface of the rotating member are worn, states of the pressing surface and the friction surface change due to the wear. This considerably changes the rotation resistance acting upon the main shaft. As a result, as the wear progresses, pulsation occurs again in the rotation of the main shaft, thereby increasing the surface roughness of the finishing surface of the workpiece.
Therefore, when the workpiece is rotated and processed while the brake device applies a constant rotation resistance to the main shaft, in order to reduce the wear, it is necessary to lubricate the pressing surface of the brake shoe and the friction surface of the rotating member. Although the purpose discussed in the aforementioned Patent Document 1 differs from the purpose of suppressing wear, it discusses that the main shaft drive (lathe) is provided with lubricant supplying means for supplying a lubricant to the brake device provided at the main shaft.
However, in the lubricant supplying means discussed in the aforementioned Patent Document 1, the lubricant is supplied to the pressing surface of the brake shoe and the friction surface of the rotating member by causing drops of the lubricant to fall from nozzle holes. Therefore, it is difficult for the lubricant to reach a portion where the brake shoe press-contacts the rotating member due to, for example, the influence of the viscosity of the lubricant and the centrifugal force generated by the rotation of the rotating member. Therefore, the pressing surface and the friction surface are not sufficiently lubricated, as a result of which oil films run out. Consequently, the pressing surface of the brake shoe and the friction surface of the rotating member are worn. As a result, wear cannot be sufficiently suppressed.